HD radio is the combination of a traditional analog signal with a mixed digital audio stream. The digital audio stream is typically produced separately from the analog via a linear solid-state transmitter. The output of this digital transmitter is then combined with the output of the original analog transmitter (tube type or solid state). From the beginning of this technology the challenge has been to develop an efficient method to combine these two signals.
The current and typical transmitter used today is not able to carry both the analog and digital signals at the same time. The digital signal requires additional bandwidth, which must be linear to pass the two signals. Traditional tube type transmitters are not capable of passing this type of signal without severely overheating. Newer solid-state transmitters can be modified to pass both signals simultaneously, however the maximum amount of power capable from these transmitters is still very limited because of the high cost. Other methods of transmitting both signals had to be found for stations needing high power outputs.
The first obvious method was to use two transmitters, one for the digital signal, and one for the analog. Combining two transmitters works well when the outputs of both transmitters are in phase with each other and of equal power. In HD combining, the digital signal is much lower in power, and not locked in phase with the analog signal. This creates a “mis-match” problem resulting in high amounts of wasted power. The current methods of combining are briefly described below.
High Level Combining (Separate Amplification) is Shown in FIG. 1(Prior Art)
Referring to FIG. 1, the output of the “main” Analog transmitter 2 is combined through a 10 dB (decibel) hybrid combiner 4 with the output of the Linear Digital transmitter 6.
The program audio is transmitted to the tower site (not shown) via a linear AES digital STL 8 and fed into an In Band On Channel (IBOC) exciter 10. Audio from IBOC exciter 10 is looped through the digital processor 12 to tailor the sound for the digital transmission. The exciter 10 also sends out a delayed AES stream 14 to feed the Analog Signal Audio Processor 16, which in turn feeds the input of the Analog Exciter 18.
Each of the two exciters (IBOC 10 and Analog 18) have very low power radio frequency (RF) outputs which feed the appropriate transmitters. The purpose of the transmitters is to amplify the low power of the exciter output into the high power output needed to broadcast through the antenna 20. This power level can commonly be 15,000 to 50,000 watts. The outputs of the two transmitters are then connected through a combiner hybrid 4. The main output feeds the antenna. In any combining system like this there is some of the signal that does not get to the antenna. It can be thought of as backpressure through a water hose. To prevent this back pressure from damaging the transmitter, a “release valve” or reject port 22 is used to channel this lost power into a resistive or reject load 24 which absorbs the power and converts it into heat.
The down side of this High Level combining method is low efficiency. In this arrangement, 10% of the RF signal from the Analog transmitter 2 is lost and sent to the reject load 24, while 90% of the RF signal of the IBOC transmitter 6 is lost into this same load. To compensate for the power losses, the output of the Analog transmitter 2 must be increased. This can be a hardship for a station owner if this transmitter is already operating at maximum power. The transmitter would have to be replaced with a higher power model. Also, because of the inefficiency, the electric power consumption will increase dramatically increasing the monthly power bills.
The placement of the reject load 24 must also be considered. If placed inside of the building, the heat dissipation will be such that the heating, ventilation and air conditioning (HVAC) system will need to be re-evaluated. In many cases, new larger air conditioners will be necessary to overcome the heat produced. Another option would be to purchase a reject load capable of being placed outside to remove the heat from the room. This installation would incur additional costs to cut and seal holes through the outside wall of the building, and possibly to pour a cement pad to place the reject load on. Fencing would also be recommended as the heat and RF energy in the reject load could be considered dangerous to touch.
Low Level Combining (Common Amplification) is Shown in FIG. 2 (Prior Art).
FIG. 2 shows a much more efficient version of HD radio transmission and is used when the total amount of power needed from the system is low. As in the High Level combining system, the digital audio 30 is fed into both the digital exciter 32 and analog exciter 34. Since a single affordable solid-state transmitter can accommodate the total power output necessary, the combining in a low-level combiner 36 can actually take place at the outputs of the exciters. While there are similar losses still, they exist in power levels of 50 watts or less and are insignificant. The combined analog/digital signal 38 is then fed into the solid state, linear transmitter 40 where the power can be amplified to power levels around 7,000 watts.
While this version is much more efficient and eliminates the high power losses going into a reject load, the output power levels are very restricted. For stations operating with transmitter power outputs (TPO) of 7 kW and under, this method is very attractive. Unfortunately most FM radio stations operate with much higher TPO's and the solid-state transmitters to accomplish this become extremely expensive, and for the most part not cost effective.
There is another prior art technique of combining currently proposed that utilizes two separate antennas as shown in FIG. 3. The method in FIG. 3 has just been approved for use by the Federal Communications Commission (FCC). The FIG. 3 embodiment allows the two signals (Analog 50 and Digital 52) to combine in free air space. It is efficient in that no power is lost, however it requires a second antenna 54 to be installed on the tower which will cost in tower space rental, and in many cases, the extra space is not available. There will also be a difference in signal coverage between the two signals as the two antennas are not in the same location on the tower.
Thus, the need exists for solutions to the above problems with the prior art.